Complete BmFib-L knockout reveals its indispensable role in silk fiber formation

Silkworm (Bombyx mori), belonging to the order Lepidoptera, is an important model insect for economic and scientific research. The capacity of the silkworm to secrete robust silk renders it a valuable economic resource, while its biological characteristics offer insights into a number of scientific disciplines. Despite the extensive research conducted to elucidate the mechanisms of silk secretion, many aspects remain unclear. In this study, a mutant strain was generated by editing the BmFib-L gene for silk fibroin, BmFib-L was observed to terminate prematurely at 27 amino acids, using the CRISPR/Cas9 system. The BmFib-L was entirely absent in the mutant strain in comparison to the wild type (WT). On the third day of the fifth instar, the posterior silk glands of the mutant strain were dissected and found to have no significant morphological differences compared to the WT. However, the mutant cocoon phenotypes exhibited significant changes. The findings demonstrated that BmFib-L plays a pivotal role in silk formation, yet is not indispensable for silk secretion. Furthermore, BmFib-L exerts a discernible influence on the mechanical properties of silk. A comparative analysis of the transcriptome of the mutant strain and the WT revealed that the elevated expression of transporter and metabolism-related genes in the mutant strain preserved the equilibrium of the silk gland cells, thereby guaranteeing the typical development of the mutant silk gland cells. These findings offer a novel perspective for investigating the mechanism of silk secretion in silkworms.